Advanced portable identification devices such as passive entry and immobilizer keys for vehicles have an analog three-dimensional front end, which is an integrated circuit, including three independent receiver channels each intended to be connected to one out of three external antenna circuits included in the device. The three antenna circuits have antenna structures that are physically oriented at mutually 90°. With such a device, signals from a transceiver in a vehicle are detected independent of orientation in space of the device. Typically, these devices operate at a frequency in a low frequency (LF) range around 125 kHz or 135 kHz. For an extended range, they are battery-powered.
Regarding passive entry systems, noise issues have led to proposals for much lower operating frequencies just at the limit of human perception, i.e. around 20 kHz. Changing an analog three-dimensional front end to such a very low frequency is possible, but existing immobilizer concepts still work in the low frequency (LF) range (around 125 kHz or 135 kHz). With a front end operating at a very low frequency (VLF) for the passive entry functionality and at a low frequency (LF) for the immobilizer functionality, a fourth antenna circuit tuned to the low frequency would be required.